In brief:
Achilles tendinitis.

When the achilles tendon is relaxed, such as when sitting, or waking up in the morning, the blood flow is lessened, resulting in pain.

As you exercise the area by walking, or participating in more strenuous activities, the blood flow increases, warms the area, and, makes it feel better. You may need physical therapy to resolve the issue.

In brief:
Achilles tendinitis.

When the achilles tendon is relaxed, such as when sitting, or waking up in the morning, the blood flow is lessened, resulting in pain.

As you exercise the area by walking, or participating in more strenuous activities, the blood flow increases, warms the area, and, makes it feel better. You may need physical therapy to resolve the issue.
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